Mimetic Gravity Description of Loop Quantum Cosmology: Reinterpreting the Bounce and Inflation from a Curvature Perspective
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Arxiv:2010.15629V2 [Gr-Qc] 10 Mar 2021 Gravity Models Contents
Prepared for submission to JHEP Supersymmetric minisuperspace models in self-dual loop quantum cosmology K. Eder,1 H. Sahlmann,2 Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Institute for Quantum Gravity (IQG), Staudtstr. 7,D-91058 Erlangen, Germany E-mail: [email protected], [email protected] Abstract: In this paper, we study a class of symmetry reduced models of N = 1 super- gravity using self-dual variables. It is based on a particular Ansatz for the gravitino field as proposed by D’Eath et al. We show that the essential part of the constraint algebra in the classical theory closes. In particular, the (graded) Poisson bracket between the left and right supersymmetry constraint reproduces the Hamiltonian constraint. For the quantum theory, we apply techniques from the manifestly supersymmetric approach to loop quantum supergravity, which yields a graded analog of the holonomy-flux algebra and a natural state space. We implement the remaining constraints in the quantum theory. For a certain subclass of these models, we show explicitly that the (graded) commutator of the supersymmetry constraints exactly reproduces the classical Poisson relations. In particular, the trace of the commutator of left and right supersymmetry constraints reproduces the Hamilton con- straint operator. Finally, we consider the dynamics of the theory and compare it to a quantization using standard variables and standard minisuperspace techniques. Keywords: Cosmology of Theories beyond the SM, Models of Quantum Gravity, Super- -
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Arxiv:0804.0672V2 [Gr-Qc]
Quantum Cosmology Claus Kiefer1 and Barbara Sandh¨ofer2 1 Institut f¨ur Theoretische Physik, Universit¨at zu K¨oln, Z¨ulpicher Straße 77, 50937 K¨oln, Germany. [email protected] 2 Institut f¨ur Theoretische Physik, Universit¨at zu K¨oln, Z¨ulpicher Straße 77, 50937 K¨oln, Germany. [email protected] Summary. We give an introduction into quantum cosmology with emphasis on its conceptual parts. After a general motivation we review the formalism of canonical quantum gravity on which discussions of quantum cosmology are usually based. We then present the minisuperspace Wheeler–DeWitt equation and elaborate on the problem of time, the imposition of boundary conditions, the semiclassical approxi- mation, the origin of irreversibility, and singularity avoidance. Restriction is made to quantum geometrodynamics; loop quantum gravity and string theory are discussed in other contributions to this volume. To appear in Beyond the Big Bang, edited by R. Vaas (Springer, Berlin, 2008). Denn wo keine Gestalt, da ist keine Ordnung; nichts kommt, nichts vergeht, und wo dies nicht geschieht, da sind ¨uberhaupt keine Tage, kein Wechsel von Zeitr¨aumen. Augustinus, Bekenntnisse, 12. Buch, 9. Kapitel 1 Why quantum cosmology? Quantum cosmology is the application of quantum theory to the universe as a whole. At first glance, this may be a purely academic enterprise, since quan- arXiv:0804.0672v2 [gr-qc] 22 Apr 2008 tum theory is usually considered to be of relevance only in the micoroscopic regime. And what is more far remote from this regime than the whole uni- verse? This argument is, however, misleading. -
Arxiv:Gr-Qc/0601085V1 20 Jan 2006 Loop Quantum Cosmology
AEI–2005–185, IGPG–06/1–6 gr–qc/0601085 Loop Quantum Cosmology Martin Bojowald∗ Institute for Gravitational Physics and Geometry, The Pennsylvania State University, 104 Davey Lab, University Park, PA 16802, USA and Max-Planck-Institute for Gravitational Physics Albert-Einstein-Institute Am M¨uhlenberg 1, 14476 Potsdam, Germany Abstract Quantum gravity is expected to be necessary in order to understand situations where classical general relativity breaks down. In particular in cosmology one has to deal with initial singularities, i.e. the fact that the backward evolution of a classical space-time inevitably comes to an end after a finite amount of proper time. This presents a breakdown of the classical picture and requires an extended theory for a meaningful description. Since small length scales and high curvatures are involved, quantum effects must play a role. Not only the singularity itself but also the sur- rounding space-time is then modified. One particular realization is loop quantum cosmology, an application of loop quantum gravity to homogeneous systems, which arXiv:gr-qc/0601085v1 20 Jan 2006 removes classical singularities. Its implications can be studied at different levels. Main effects are introduced into effective classical equations which allow to avoid interpretational problems of quantum theory. They give rise to new kinds of early universe phenomenology with applications to inflation and cyclic models. To resolve classical singularities and to understand the structure of geometry around them, the quantum description is necessary. Classical evolution is then replaced by a difference equation for a wave function which allows to extend space-time beyond classical sin- gularities. -
Loop Quantum Cosmology, Modified Gravity and Extra Dimensions
universe Review Loop Quantum Cosmology, Modified Gravity and Extra Dimensions Xiangdong Zhang Department of Physics, South China University of Technology, Guangzhou 510641, China; [email protected] Academic Editor: Jaume Haro Received: 24 May 2016; Accepted: 2 August 2016; Published: 10 August 2016 Abstract: Loop quantum cosmology (LQC) is a framework of quantum cosmology based on the quantization of symmetry reduced models following the quantization techniques of loop quantum gravity (LQG). This paper is devoted to reviewing LQC as well as its various extensions including modified gravity and higher dimensions. For simplicity considerations, we mainly focus on the effective theory, which captures main quantum corrections at the cosmological level. We set up the basic structure of Brans–Dicke (BD) and higher dimensional LQC. The effective dynamical equations of these theories are also obtained, which lay a foundation for the future phenomenological investigations to probe possible quantum gravity effects in cosmology. Some outlooks and future extensions are also discussed. Keywords: loop quantum cosmology; singularity resolution; effective equation 1. Introduction Loop quantum gravity (LQG) is a quantum gravity scheme that tries to quantize general relativity (GR) with the nonperturbative techniques consistently [1–4]. Many issues of LQG have been carried out in the past thirty years. In particular, among these issues, loop quantum cosmology (LQC), which is the cosmological sector of LQG has received increasing interest and has become one of the most thriving and fruitful directions of LQG [5–9]. It is well known that GR suffers singularity problems and this, in turn, implies that our universe also has an infinitely dense singularity point that is highly unphysical. -
Equivalence of Models in Loop Quantum Cosmology and Group Field Theory
universe Article Equivalence of Models in Loop Quantum Cosmology and Group Field Theory Bekir Bayta¸s,Martin Bojowald * and Sean Crowe Institute for Gravitation and the Cosmos, The Pennsylvania State University, 104 Davey Lab, University Park, PA 16802, USA; [email protected] (B.B.); [email protected] (S.C.) * Correspondence: [email protected] Received: 29 November 2018; Accepted: 18 January 2019; Published: 23 January 2019 Abstract: The paradigmatic models often used to highlight cosmological features of loop quantum gravity and group field theory are shown to be equivalent, in the sense that they are different realizations of the same model given by harmonic cosmology. The loop version of harmonic cosmology is a canonical realization, while the group-field version is a bosonic realization. The existence of a large number of bosonic realizations suggests generalizations of models in group field cosmology. Keywords: loop quantum cosmology; group field theory; bosonic realizations 1. Introduction Consider a dynamical system given by a real variable, V, and a complex variable, J, with Poisson brackets: fV, Jg = idJ , fV, J¯g = −idJ¯ , fJ, J¯g = 2idV (1) d −1 −1 ¯ for a fixed real d. We identify Hj = d ImJ = −i(2d) (J − J) as the Hamiltonian of the system and interpret V as the volume of a cosmological model. The third (real) variable, ReJ, is not independent provided we fix the value of the Casimir R = V2 − jJj2 of the Lie algebra su(1, 1) given by the brackets (1). To be specific, we will choose R = 0. Writing evolution with respect to some parameter j, the equations of motion are solved by: V(j) = A cosh(dj) − B sinh(dj) (2) ReJ(j) = A sinh(dj) − B cosh(dj) . -
Conformally Coupled General Relativity
universe Article Conformally Coupled General Relativity Andrej Arbuzov 1,* and Boris Latosh 2 ID 1 Bogoliubov Laboratory for Theoretical Physics, JINR, Dubna 141980, Russia 2 Dubna State University, Department of Fundamental Problems of Microworld Physics, Universitetskaya str. 19, Dubna 141982, Russia; [email protected] * Correspondence: [email protected] Received: 28 December 2017; Accepted: 7 February 2018; Published: 14 February 2018 Abstract: The gravity model developed in the series of papers (Arbuzov et al. 2009; 2010), (Pervushin et al. 2012) is revisited. The model is based on the Ogievetsky theorem, which specifies the structure of the general coordinate transformation group. The theorem is implemented in the context of the Noether theorem with the use of the nonlinear representation technique. The canonical quantization is performed with the use of reparametrization-invariant time and Arnowitt– Deser–Misner foliation techniques. Basic quantum features of the models are discussed. Mistakes appearing in the previous papers are corrected. Keywords: models of quantum gravity; spacetime symmetries; higher spin symmetry 1. Introduction General relativity forms our understanding of spacetime. It is verified by the Solar System and cosmological tests [1,2]. The recent discovery of gravitational waves provided further evidence supporting the theory’s viability in the classical regime [3–6]. Despite these successes, there are reasons to believe that general relativity is unable to provide an adequate description of gravitational phenomena in the high energy regime and should be either modified or replaced by a new theory of gravity [7–11]. One of the main issues is the phenomenon of inflation. It appears that an inflationary phase of expansion is necessary for a self-consistent cosmological model [12–14]. -
Ads₄/CFT₃ and Quantum Gravity
AdS/CFT and quantum gravity Ioannis Lavdas To cite this version: Ioannis Lavdas. AdS/CFT and quantum gravity. Mathematical Physics [math-ph]. Université Paris sciences et lettres, 2019. English. NNT : 2019PSLEE041. tel-02966558 HAL Id: tel-02966558 https://tel.archives-ouvertes.fr/tel-02966558 Submitted on 14 Oct 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Prepar´ ee´ a` l’Ecole´ Normale Superieure´ AdS4/CF T3 and Quantum Gravity Soutenue par Composition du jury : Ioannis Lavdas Costas BACHAS Le 03 octobre 2019 Ecole´ Normale Superieure Directeur de These Guillaume BOSSARD Ecole´ Polytechnique Membre du Jury o Ecole´ doctorale n 564 Elias KIRITSIS Universite´ Paris-Diderot et Universite´ de Rapporteur Physique en ˆIle-de-France Crete´ Michela PETRINI Sorbonne Universite´ President´ du Jury Nicholas WARNER University of Southern California Membre du Jury Specialit´ e´ Alberto ZAFFARONI Physique Theorique´ Universita´ Milano-Bicocca Rapporteur Contents Introduction 1 I 3d N = 4 Superconformal Theories and type IIB Supergravity Duals6 1 3d N = 4 Superconformal Theories7 1.1 N = 4 supersymmetric gauge theories in three dimensions..............7 1.2 Linear quivers and their Brane Realizations...................... 10 1.3 Moduli Space and Symmetries............................ -
Loop Quantum Cosmology and CMB Anomalies
universe Article Cosmic Tangle: Loop Quantum Cosmology and CMB Anomalies Martin Bojowald Institute for Gravitation and the Cosmos, The Pennsylvania State University, 104 Davey Lab, University Park, PA 16802, USA; [email protected] Abstract: Loop quantum cosmology is a conflicted field in which exuberant claims of observability coexist with serious objections against the conceptual and physical viability of its current formulations. This contribution presents a non-technical case study of the recent claim that loop quantum cosmology might alleviate anomalies in the observations of the cosmic microwave background. Keywords: loop quantum cosmology; observations “Speculation is one thing, and as long as it remains speculation, one can un- derstand it; but as soon as speculation takes on the actual form of ritual, one experiences a proper shock of just how foreign and strange this world is.” [1] 1. Introduction Quantum cosmology is a largely uncontrolled and speculative attempt to explain the origin of structures that we see in the universe. It is uncontrolled because we do not have a complete and consistent theory of quantum gravity from which cosmological models could Citation: Bojowald, M. Cosmic be obtained through meaningful restrictions or approximations. It is speculative because Tangle: Loop Quantum Cosmology we do not have direct observational access to the Planck regime in which it is expected to and CMB Anomalies. Universe 2021, be relevant. 7, 186. https://doi.org/10.3390/ Nevertheless, quantum cosmology is important because extrapolations of known universe7060186 physics and observations of the expanding universe indicate that matter once had a density as large as the Planck density. -
Cosmologies of Extended Massive Gravity
Cosmologies of extended massive gravity Kurt Hinterbichler, James Stokes, and Mark Trodden Center for Particle Cosmology, Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA (Dated: June 15, 2021) We study the background cosmology of two extensions of dRGT massive gravity. The first is variable mass massive gravity, where the fixed graviton mass of dRGT is replaced by the expectation value of a scalar field. We ask whether self-inflation can be driven by the self-accelerated branch of this theory, and we find that, while such solutions can exist for a short period, they cannot be sustained for a cosmologically useful time. Furthermore, we demonstrate that there generally exist future curvature singularities of the “big brake” form in cosmological solutions to these theories. The second extension is the covariant coupling of galileons to massive gravity. We find that, as in pure dRGT gravity, flat FRW solutions do not exist. Open FRW solutions do exist – they consist of a branch of self-accelerating solutions that are identical to those of dRGT, and a new second branch of solutions which do not appear in dRGT. INTRODUCTION AND OUTLINE be sustained for a cosmologically relevant length of time. In addition, we show that non-inflationary cosmological An interacting theory of a massive graviton, free of solutions to this theory may exhibit future curvature sin- the Boulware-Deser mode [1], has recently been discov- gularities of the “big brake” type. ered [2, 3] (the dRGT theory, see [4] for a review), al- In the second half of this letter (which can be read lowing for the possibility of addressing questions of in- independently from the first), we consider the covariant terest in cosmology. -
Milgrom's Law and Lambda's Shadow: How Massive Gravity
Journal of the Korean Astronomical Society preprint - no DOI assigned 00: 1 ∼ 4, 2015 June pISSN: 1225-4614 · eISSN: 2288-890X The Korean Astronomical Society (2015) http://jkas.kas.org MILGROM'S LAW AND Λ'S SHADOW: HOW MASSIVE GRAVITY CONNECTS GALACTIC AND COSMIC DYNAMICS Sascha Trippe Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea; [email protected] Received March 11, 2015; accepted June 2, 2015 Abstract: Massive gravity provides a natural solution for the dark energy problem of cosmology and is also a candidate for resolving the dark matter problem. I demonstrate that, assuming reasonable scaling relations, massive gravity can provide for Milgrom’s law of gravity (or “modified Newtonian dynamics”) which is known to remove the need for particle dark matter from galactic dynamics. Milgrom’s law comes with a characteristic acceleration, Milgrom’s constant, which is observationally constrained to a0 1.1 10−10 ms−2. In the derivation presented here, this constant arises naturally from the cosmologically≈ × required mass of gravitons like a0 c√Λ cH0√3ΩΛ, with Λ, H0, and ΩΛ being the cosmological constant, the Hubble constant, and the∝ third cosmological∝ parameter, respectively. My derivation suggests that massive gravity could be the mechanism behind both, dark matter and dark energy. Key words: gravitation — cosmology — dark matter — dark energy 1. INTRODUCTION On smaller scales, the dynamics of galaxies is in ex- cellent agreement with a modification of Newtonian dy- Modern standard cosmology suffers from two criti- namics (the MOND paradigm) in the limit of small ac- cal issues: the dark matter problem and the dark celeration (or gravitational field strength) g, expressed energy problem. -
An Introduction to Loop Quantum Gravity with Application to Cosmology
DEPARTMENT OF PHYSICS IMPERIAL COLLEGE LONDON MSC DISSERTATION An Introduction to Loop Quantum Gravity with Application to Cosmology Author: Supervisor: Wan Mohamad Husni Wan Mokhtar Prof. Jo~ao Magueijo September 2014 Submitted in partial fulfilment of the requirements for the degree of Master of Science of Imperial College London Abstract The development of a quantum theory of gravity has been ongoing in the theoretical physics community for about 80 years, yet it remains unsolved. In this dissertation, we review the loop quantum gravity approach and its application to cosmology, better known as loop quantum cosmology. In particular, we present the background formalism of the full theory together with its main result, namely the discreteness of space on the Planck scale. For its application to cosmology, we focus on the homogeneous isotropic universe with free massless scalar field. We present the kinematical structure and the features it shares with the full theory. Also, we review the way in which classical Big Bang singularity is avoided in this model. Specifically, the spectrum of the operator corresponding to the classical inverse scale factor is bounded from above, the quantum evolution is governed by a difference rather than a differential equation and the Big Bang is replaced by a Big Bounce. i Acknowledgement In the name of Allah, the Most Gracious, the Most Merciful. All praise be to Allah for giving me the opportunity to pursue my study of the fundamentals of nature. In particular, I am very grateful for the opportunity to explore loop quantum gravity and its application to cosmology for my MSc dissertation. -
Massive Supergravity
1 Massive Supergravity Taichiro Kugo Maskawa Institute, Kyoto Sangyo University Nov. 8 { 9, 2014 第4回日大理工・益川塾連携 素粒子物理学シンポジウム in collaboration with Nobuyoshi Ohta Kinki University 2 1 Introduction Cosmological Constant Problem: Higgs Condensation ∼ ( 100 GeV )4 QCD Chiral Condensation ∼ ( 100 MeV )4 (1) These seem not contributing to the Cosmological Constant! =) Massive Gravity: an idea toward resolving it However, Massive Gravity has its own problems: • van Dam-Veltman-Zakharov (vDVZ) discontinuity Its m ! 0 limit does not coincides with the Einstein gravity. • Boulware-Deser ghost − |{z}10 |(1{z + 3)} = 6 =|{z} 5 + |{z}1 (2) hµν i N=h00;N =h0i massive spin2 BD ghost We focus on the BD ghost problem here. 3 In addition, we believe that any theory should eventually be made super- symmetric, that is, Supergravity (SUGRA). This may be of help also for the problem that the dRGT massive gravity allows no stable homogeneous isotropic universe solution. In this talk, we 1. explain the dRGT theory 2. massive supergravity 2 Fierz-Pauli massive gravity (linearized) Einstein-Hilbert action p LEH = −gR (3) [ ] h i 2 L L −m 2 − 2 = EH + (hµν ah ) (4) quadratic part in hµν | 4 {z } Lmass = FP (a = 1) gµν = ηµν + hµν (5) 4 In Fierz-Pauli theory with a = 1, there are only 5 modes describing properly massive spin 2 particle. : ) No time derivative appears for h00; h0i in LEH !LEH is linear in N; Ni. Lmass ∼ If a = 1, the mass term FP is also clearly linear in N h00 ! =) • Ni can be solved algebraically and be eliminated.